Alzheimer's disease (AD) is characterized by the presence of amyloid plaques mainly consisting of hydrophobic -amyloid peptide (A) aggregates and neurofibrillary tangles (NFTs) composed principally of hyperphosphorylated tau. A oligomers have been described as the earliest effectors to negatively affect synaptic structure and plasticity in the affected brains, and cellular prion protein (PrPC) has been proposed as receptor for these oligomers. The most widely accepted theory holds that the toxic effects of A are upstream of change in tau, a neuronal microtubule-associated protein that promotes the polymerization and stabilization of microtubules. However, tau is considered decisive for the progression of neurodegeneration, and indeed tau pathology correlates well with clinical symptoms such as dementia. Different pathways can lead to abnormal phosphorylation, and, as a consequence, tau aggregates into Paired Helical Filaments (PHF) and later on into NFTs. Reported data suggest a regulatory tendency of PrPC expression in the development of AD, and a putative relationship between PrPC and tau processing is emerging. However the role of tau/PrPC interaction in AD is poorly understood. In this study we show increased susceptibility to A derived diffusible ligands (ADDLs) in neuronal primary cultures from PrPC knock-out mice, compared to wild-type, which correlates with increased tau expression. Moreover, we found increased PrPC expression that paralleled with tau at early ages in an AD murine model, and in early Braak stages of AD in affected individuals. Taken together, these results suggest a protective role for PrPC in AD by down-regulating tau expression, and they point to this protein as being crucial in the molecular events that lead to neurodegeneration in AD.